A Split-Single Two-Stroke EngineAn efficienttunit for propelling a class " C "

Hydroplane

by R. E. Mitchell

TH E American design and British copiesof what has now become known as the

they show that at the lower speeds the engines

“ modern racing engine ” suffers from one majorfitted with ringed light alloy pistons show a

defect. It is unreliable. No doubt the high outputsurprising lack of power. It may be argued

figures at speeds of the order of 15,000 r.p.m.that full-size racing engines show a similar ten-

are realised on the test bench where the loaddency but here drivers and gearboxes can ensure

may not be applied until these high speeds arethat the r.p.m. are kept high. These engines also

reached and the ietstart with comparative ease. It is nossible that

adjusted in order ‘togive the maximumoutput.

When one ofthese engines is putinto a car or a boat,the majority cannotbe persuaded togive of their best.The rules governingmodel car racingallow as many lapsstart as the ownerdesires to attainmaximum speed,or if the engine isparticularly slow offthe mark the carmay be lead manu-ally by means of thetethering cable. Inboating circles con-ditions are consider-ably more difficult.Half a lap (i.e.. .

needle can be very carefully the poor startability and low

- - - - RINGLESS CAST-iRON PISTON

RINGED AL. ALLOY PISTON

50 yards) is allthat the rule allows before timing begins. Unlikecars, the highest demand on the engine is at thestart where the propeller blades are working atan unfavourable angle of attack and the hydro-plane is behaving as a displacement boat. Toavoid this the technique appears to let the enginerace at very high speed, which can only do itconsiderable harm if not wreck it altogetherand hurl the boat forward hoping that the enginewill maintain its high revolutions. More oftenthan not the launch is faulty or the surface of thewater is blamed for the fact that the revs. dodrop and the engine splutters its way round thecourse or fails to complete the distance. Startingthis type of engine also appears to be extremelydifficult and is possibly the reason why mechanicalstarters have become popular.

speed characteristicsare due to thealuminium alloypiston’s inability toseal effectively thetransfer port fromthe exhaust port,at least when theengine is relativelycool. The portshave crept furtherand further roundthe cylinder peri-phery to increasethe power at thetop end of the scale,losing sight of thefact that the enginehas to attain theserevs. to be of anyuse. The lack ofpower is not ap-parent in enginesfitted with solidsteel or cast-ironpistons because herethe piston clearance

SPEED

Fig. 1. Power outputs of plain cast-iron piston, and ringedlight-alloy piston

is of the order of one-tenth of that which has tobe given to aluminium alloy pistons to allow forexpansion at the working temperature. Theclearance allowed for the rotary inlet valve isnot such a bad offender because this is moreeffectively sealed by the liquid fuel/oil mixture.A fact which supports this theory is that an engineof this type is easier to start with glow-plugthan if spark-ignition is employed. With spark-ignition, by the time there is enough fuel and oilto effectively seal the ports the plug electrodeshave become drowned so that no spark takesplace. At M.P.B.A. regattas silencers are com-pulsory and these are objected to by some usersof this type of engine because it is more difficultto get rid of the excess fuel after flooding so thatthe process may be repeated. Also, the familiarexpedient of priming through the exhaust portcannot be done so easily in these circumstances.

The starting difficulty is not due to an over-rich mixture since with methanol (the most usualfuel) the correct mixture is obtained at 65 deg. F.while the lowest combustible mixture occurs at

If the published b.h.p. curves of the engines inquestion are examined they only start at about8,000 r.p.m. and when they are produced to meetzero they give a complete curve of the type shownin Fig. I. Compared with the curves publishedfor lapped piston engines, also shown in Fig. I,

471

THE M ODEL ENGINEER

45 deg. F. A glow plug, on the other hand, will of the scale have been improved.continue to glow even in such wet circumstancesand the engine will consequently start easier.Starting the car on its run appears to be doneon an over-rich mixture and it is allowed to rununtil the mixture has weakened by which timethe speed may have been built up. It is possiblethat the high centrifugal forces, which are setup on small circular tracks, acting on the fuellevel, thus giving a large variation in mixture

The engineII and builtreferred to is that fitted to Sparky

by Mr. G. Lines, of Orpington.To readers of Mr. Westbury and regatta

reports in TH E M O D E L E N G I N E E R since theClass “ C ” (10c.c.) hydroplane was introduced,it will be noted that this type of craft is usuallyextremely unstable. It has been argued that thehigh power/weight ratio is responsible. Underthe usual regatta conditions obtaining in this

l

Fig. 2. Longitudinal section through transfer cylinder and crankshaft ; full size

strength, may be a decided advantage. Some-where during the run the mixture is slightly onthe rich side to give the maximum power outputso enabling the quarter mile or so to be timed.In the hydroplane field the centrifugal force isconsiderably lower and its effect is not so pro-nounced, with the result that the whole run ismost likely done with a very rich mixtureand maximum power is never attained. A boatwill occasionally run out of fuel a lap or so afterthe timed distance and, in some instances, thespeed will increase considerably for the lastfifty yards or so immediately before the fueltank is empty.

It is quite likelv that such a boat would havepetered but had the mixture been weakened atthe start. The overall performance of one well-known hydroplane engine has been improvedout of all recognition by replacing the alurniniumalloy piston with rings by a piston skirt of cast-iron minus the rings. It is more than likely thatthe maximum output has been reduced by anincrease in the mechanical friction, but verydefinitely output and reliability at the lower end

472

country this is not the case. It is not certain whatthe effect of the weight of the boat is on the powerrequired to drive it.

The Class “ B ” hydroplanes, Beta and BetaIZ, were powered by the same o.h.v. four-strokeengine which, when first assembled, had a poweroutput of slightly over 1.5 h.p. It is quite possiblethat with subsequent running this has increasedsomewhat. Both these boats were fairly heavyfor their class, being of the order of 8-1/4 lb.and were propelled fairly consistently in almostany conditions at about 45 m.p.h. The averageClass “ C ” hydroplane weighs no more thanhalf this figure, but speeds at regattas above45 m.p.h. are a rarity. Therefore their enginescould not have been developing their full outputof 1-1/4 h.p. + of which they are claimed to becapable. There is such a wide variety of hulldesign that the hull cannot be held responsible.Under competitive conditions the roughnessof the water is the same for all classes of boatsand obviously the heavier the boat the morestable it can be expected to behave. The intro-duction of surface propellers has also increased

APRIL IO, 1952

the instability of hulls, but this has been more and one combustion chamber are used. Thepronounced in Class “ C ” because since the intention, originally, was to use two carburettors ;boat is inherently more unstable the propellerleaves the water more frequently. This means

but this was not considered very practical, so

that the torque reaction on the hull and the sideinstead, a manifold connecting the two inletports is used. It varies from the more usual

thrust on the stern, obtained by the use of surface design of this type of engine in that there arepropellers, are alternatively applied and removedas the hull bounces in a vertical plane.

two crankshafts which are synchronised by makingAlso, the crank discs a pair of spur wheels. Internal

when the propeller is thrown clear of the water gearing is preferable since general enginethe thrust is removed which further increases the “ petroil ” lubrication can take care of them.

ize Fig. 3. Transverse section through both cylinders ; full size_i

vertical instability. This type of instability canbe minimised by the careful attention to theplaning angles (most appear to fit planes at toosteep an angle) and the position of the centre ofgravity. It follows that a large contribution tostability could be made by the removal of sidethrust on the stern and the torque reaction onthe hull. The only method of eliminating thoseeffects would be to fit two propellers runningin opposite directions. Any method of trans-mitting the power to two shafts through gearsfrom one engine was considered to be tooinefficient. Using two engines with synchronisinggears on the flywheels was considered. Sinceeach engine would be of 5 c . c . capacity and almostcertainly a two-stroke in this size it would probablysuffer from the faults previously assigned to thistype of engine. A convenient way out was foundin which essentiallv two engines are used, and isa variation of a principle used in several full-sizedesigns. The general arrangement of the engineis shown in Figs. 2 and 3.

It consists of duplicating everything exceptthat only one transfer passage, one exhaust port

The slight displacement of the pistons, in whichone controls the transfer port while the othertakes care of the exhaust can be convenientlyachieved by orienting the gear teeth differentlyon the crank discs. Choosing a suitable numberof teeth governed by the angle of displacementrequired and displacing the wheels by one toothwil! give a similar effect. If the constructormakes his own gearcutting tools a non-standarddiametral pitch can easily be used, and this wasthe method adopted in the present case. A usualtiming of port events for a two-stroke may beto open the exhaust port 65 deg. before bottomdead centre while the transfer opens 55 deg.before b.d.c.

The closing takes place at similar angulardisplacements after b.d.c. This type of timingis shown in Fig. 8. With the simple two-strokeit is impossible to vary the symmetry of thetiming which is not the most efficient that couldbe desired ; the transfer possibly opening toosoon after the exhaust while some charge ispossibly lost by closing the exhaust port afterthe transfer port has closed. Considerable experi-

473

THE MODEL ENGINEER

TDC

Fig. 8. Orthodox two-stroke timing

ment with deflector and cylinder-headdesign appears to be necessary if this isto be prevented. In the two piston

APRIL IO, 1952

the individual timing of the transfer and exhaustports, while a combination of the two is shown inFig. 10. The out-of-phase angle of I0deg. meansthat 36 teeth are required on the crank discs.This will enable the conventional symmetricaltiming to be used, if necessary, to compare withthe intended timing.

One difficulty which presented itself was themaking of the cylinder-head joint. This wouldbe easy if the cylinders were in one block,but with monoblock construction the provisionof adequate transfer and exhaust passageswould be very difficult. These could be securedto the outside- of the cylinders by, say, brazing,and although ports brazed to cast-iron have beenused by various constructors it is not a soundmethod, particularly where vibration is present.Clamped-on ports which have previously beenused in two-stroke design were dismissed as notbeing satisfactory to use in the present case.If cylinder liners are used, which makes thecutting of transfer and exhaust passages in thecylinder block an easy operation, the joint cannot

T.

designs where one piston is responsiblefor the transfer while the other piston EXHAUST

takes care of the exhaust, it is an easymatter to upset the symmetry by //advancing the exhaust controlling pistonby a suitable amount. Using the timing

1

given above, it was considered that acrankshaft displacement angle of IO deg.

I

would be adequate. This enables rheexhaust port to be opened 20 deg. <\before the transfer port opens whilethey both close together. This dis-placement means that both pistons are

1L-1

not at top centre together but from B

D.C

IT

\

-

61previous experience a difference of+/- 5 deg. in the ignition timing can be Fig. 9. Individual transfer and exhaust timing used for split-regarded as insignificant. Fig. 9 shows single

Fig. 10. Combined timings show in Fig. 9 used forsplit-single

474

be made on the top end face of the liners dueto the provision of a common combustionchamber being imperative.

The difficulty was overcome by making aport belt for rhe two cylinders with ports coinci-ding with those in the liners. The liners each hasa narrow flange which rests on the top of the portbelt. Over the protruding portion of the linersis placed a second belt which is provided withcooling fins, the thickness of which is slightlygreater than the upper portion of the cylinderliners. On top of this the cylindsr-head made itsjoint. It will be seen that the joint on the linersis made at the flanges between the top of theport belt and the bottom of the finned belt.This necessitated having flanges of exactly thesame depth but since the actual dimension isnot critical this was easily carried out as describedlater.

A further advantage of this design is that adeflector on the piston is not required since theincoming gas is automatically directed up thetransfer cylinder into the combustion chamber

_ ++a

T H E M O D E L E N G I N E E R , APRIL IO, 1952

and down into the exhaust cylinder. In order tohelp the charge in the correct direction both the

The rotary valves call for no comment except

transfer and exhaust piston crowns are madethat they consist of mild-steel discs on nickel

conical, the angle of which is very obtuse.steel spindles which run in dural bearings. One

Further, there is no possibility of leakage fromof the rotary valve spindles extends beyond its

the transfer to the exhaust port around the pistonbearing in order to take a coupling for a magnetodrive. The usual 180 deg. induction period

An external view of the finished enginc

skirt and the ports occupy the whole of the peri-phery of the liners except for a smail portionwhere they are adjacent, there being no spaceavailable at this spot and it is an advantage tohave the cylinder centres as close together aspossible. The actual portion of the circumferenceused for ports is go per cent. Electron pistons areused, as in all previous designs to keep thereciprocating weight to a minimum. Two com-pression rings are fitted to each piston. Sincethese pistons have to be fitted with large expansionclearances the exhaust piston is fitted with athird ring at the bottom of its skirt to preventleakage of the charge from the crankcase to theexhaust port.

It was rather a problem to decide on a suitablebore and stroke as obviously the centre distancesof the cylinders determine the bore while thestroke is governed by crankshaft centres.Actually, to avoid too large a bore/stroke ratio,the cylinders were made desaxe to the extentof 1/16 in. which E. T. Westbury has often recom-mended. The final dimensions arrived at were0.740 in. bore by 0.700 in. stroke which givesa total displacement of 9.8 c . c .

was considered to be excessive in view of thefact that the transfer port closes rather later thanthe single-cylinder engines and thus was reducedto 160 deg. The opening of both valves takesplace at the same time at which the transfer andexhaust valves close.

For a twin-screw boat the propeller shaftswould not be parallel and an attempt was madeto design the engine so that the crankshaftswould be at the same angle as the propeller shafts,thereby avoiding the use of universal joints at theinboard end. This would entail the making ofobtuse angie bevels for the synchronising gearsand complicated angular bolting faces would alsoresult. Also the rotary valves would be reducedin diameter. The straightforward method wasadopted and two universal joints per shaft areused. In order to obtain a useful flywheeldiameter it is necessary to stagger them one behindthe other and provide crankshafts of differentlengths to accommodate them. The flywheels,which are of mild-steel, are secured to the shafts.by means of taper split collets, the female portionof the universal joint acting as a nut.

(To be continued)

475

*A Split-Single Two-Stroke EngineAn efficient unit for propelling a class “ C ”

Hydroplane

by R. E. Mitchell

IT will be noted that the plug is offset towardsthe transfer cylinder. This was done, since in

this position the charge is in all probabilityless diluted with exhaust gases and consequentlywill be easier to ignite. In order to prevent gasescaping into tbe ports before the correct openingtime due to large clearances between the cylinderwall and piston, the top ring is placed as near tothe top of the piston as is thought practical andthis same distance is used between the rings.The transfer cylinder piston is machined awayfor lightening purposes on the inside, since theoutside is in contact with the transfer port andcrankcase volume is not therefore reduced as inthe case of the exhaust piston where the outsideof the piston is isolated from the crankcase.In this case the lightening is done on the outsideof the piston. Case-hardened hollow gudgeon-pins

*Continued from page 475, “ M.E.,” April 10,1952.

are used, but they are not fitted with end pads.Previous experience with end pads has shownthat they can cause severe scoring of the cylinderbore. Endwise location is secured by means ofpiano wire pins through holes drilled in bothgudgeon-pin and piston. The pins are securedby having their endsangles.

bent over at right-

The crankshafts, which are of the usual over-hung type with the crankpins sufficiently longto provide a drive for the inlet valves, are mach-ined from 3 per cent. nickel-steel and are case-hardened on completion. Care had to be exercisedin the heat treatment to prevent distortion, sinceno grinding equipment was available. Lapping ofthe main journals and crankpin to diameter wasthe only work carried out on the crankshaftsafter hardening. The shafts were drilled withthe idea of supplying oil, by means of a pump,to the big-ends at a later date.

Complete balancing was not found to be pos-

503

THE MODEL ENGINEER APRIL 17, 1952

sible owing to the fact that the periphery of thecrank discs has to be complete to receive the

been found that flat surfaces provide better

synchronising gear teeth. Actually, about one-bolting faces for additions that are sometimes

quarter of the reciprocating weight is balancedfound necessary in development work of thisnature.

instead of the usual half. In passing, completebalancing of the rotary valves was not obtained.

The first parts to be tackled were the crank-

The three flat-bottomed holes, drilled nearlyshafts which are shown in Figs. II and 12. This

through the thickness of the disc and covered bywas done because it was felt that, with the crude

thin discs of brass soldered in, hardly provedgear-cutting equipment available, the exactcentre distance of 1-3/32 in. of the crankshafts

Fig. 11. Crankshafi s an d main journal housings

sufficient and the peculiar shape of the discsin the vicinity of these holes is the result of filingaway metal to obtain as good a balance as possible.Reducing the thickness of the disc opposite tothe cut-away portion, at the expense of increasedcrankcase volume, would, most likely, have beena better method. The out-of-balance forceshere would have been less had aluminium alloydiscs been used. These were decided againstbecause trouble. in the form of severe scoring.had previously been encountered when attemptingto run dural on dural. All the light alloy parts,which include the crankcase, front and rear crank-case covers together with the stationary portionsof the inlet valves, the port belt, cooling fins,cylinder-head and carburettor, are machinedfrom the solid bar. The material is an aluminiumalloy of the duralumin type to B.S.S. 6L1obtained from Messrs. Rollett, of Liverpool.This material gives a minimum tensile strengthof 24 tons per sq. in. compared with 9 tons persa. in. for the more usual casting ahoy DTD 424.The common fault of porosity with aluminiumalloy castings is also avoided and it is doubtedwhether the making of patterns for one-offjobs i s worth while. The angular appearanceof the engine could have been avoided but it has

could not be guaranteed. Three per cent. nickelcase-hardening steel has been used previously forcrankshaft construction with excellent results.The bar was gripped in the chuck by the end to belater machined for the crankpin. It was thenreduced to 1.14 in. diameter which is the outsidediameter of the gear teeth.The journals of 3/8 in.and 1/4 in. respectively were then turned 0.001 in.oversize to allow for lapping after case-hardening.A 1/16 in. radius is provided at the comers toelminate, as far as possible, subsequent fatiguefailure. Sharp corners act as “ stress raisers,”and fatigue cracks can easily spread from suchabrupt changes in cross-sectional area. For asimilar reason all highly stressed parts are givenas good a surface finish as possible to eliminatecircumferential grooves left by the turning tool.Stub teeth, which have been used for camshaftdrive gears in previous four-stroke designs,were decided upon, using 36 teeth. A gear wheelof almost these dimensions was found and asingle point high-speed steel tool was ground,usine this wheel as a gauge. making slight modifi-cations as were thought necessary.-

The tool was mounted on its side in the lathetool holder and the teeth were planed by meansof the rack and indexing from a drilled plate

THE MODEL ENGINEER APRIL 17, 1952

Fig. 12. Details of crankshafts

secured to the end of the lathe mandrel. Theteeth were finished to within about 0.002 in.of the correct depth before passing on to the nexttooth. The final finishing was done afterwardsin one cut without moving the top-slide. Thecomponent was then reversed and the 3/8 in.diameter portion was centred by inserting in ahole of the correct size bored through a piece of1/2 in. diameter brass rod gripped in the three-jawchuck, and the chuck key was given an extraturn after inserting the crankshaft. Had colletsbeen available this operation would have beeneasier. The crankshaft was then drilled 7/64 in.diameter to provide an oil way for big-end lubri-cation. Also at this setting the portion whichhad previously gripped in the chuck was mach-ined true. The actual diameter here is not impor-

tant provided that it is larger than the enginestroke plus the crankpin diameter. At this stage,two 3/8 in. diameter holes were bored in a block.ofbrass at the correct centre distance intended forthe crankshafts. This was carried out in thevertical slide using the cross-slide index to obtainthe required centre distance. From this it wasfound that the gears worked together satisfac-torily, although the back lash was rather morethan could have been desired.

To machine the crankpins and balance weightsthe components were mounted in turn on a Keatsvee-angle plate secured to the lathe faceplate.Great care has been taken to obtain the correctorientation between the gear teeth and the crank-pin to obtain the designed 10 deg. lead of theexhaust piston. The correct eccentricity to give

c

Fig. 13. Crankcase and port belt

THE MODEL E N G I N E E R A P R I L 17, 1 9 5 2

3

0-

I3, ”

xi ,

a stroke of 0.700 in. was measured by means ofa dial gauge. The crankpin, which is 1/4 in.diameter, was a straightforward job but owingto the intermittent nature of the cuts, for the mostpart, these were only light to avoid the risk of thecrankshaft turning in the vee of the angle-plate.A 1/16 in. radius was left where the crankpin joinsthe crank disc and a 7/64 in. diameter drilledup the centre to a depth equal to the lengthrequired to drive the rotary valve. A hole of1/32 in. diameter was drilled diagonally throughthe crankpin and crank disc to connect with thecentral 7/64 in. diameter hole. After the machin-

‘ing had been completed the crankshafts werepack carburised at 920 deg. C. to give a totalcase depth of 0.010 in. Experiments were firstcarried out to determine the length of timerequired, because too great a depth results in thewhole tooth section being hard and, conse-quently, brittle. Experience of this had/beenobtained previously when the pinion driving thecamshaft of a four-stroke shed its teeth very early

in its career. Hardening was carried out byquenching in oil from 800 deg. C. from an electricmuffle. A light temper for half an hour at 150 deg.C. in an oil bath completed the heat treatment.The diameters were then finally lapped to sizeusing a split external lap of aluminium held in adie holder. The journals were made a fairly tightfit in the bores of the ball races.

The next part to be tackled was the crankcase,which is shown in Figs. 13 and 14, havingdecided that the designed centre distance of thecrankshafts has been adhered to. It was roughedout by sawing from 2 in. sq. duralumin bar.

The length was made equal to the width of thecrankcase over the holding-down lugs plus about3/8 in. The block was then machined all over

506

L

Fig. 14. Crankcase

except for the ends, making the faces as flat andsquare as possible. Four holes were drilled andtapped 4 B.A., one at each corner of one of thefaces. The reason for this was to secure the blockto a piece of 2 in. x 1/4 in. ground mild-steelbar which was, in turn, bolted to the faceplate.Four similar holes were drilled in one of theadjacent faces, to enable the block to be turnedthrough a right angle. This enabled the use of anangle-plate to be dispensed with since they arerather clumsy when used on a small lathe. The3/8 in. extra length of the block accommodatesthe tapped holes which are cut away whenfinally machining the ends of the crankcase.The two bores to take the main bearing housingswere bored in the lathe with the work on thefaceplate, being set over by the correc tamount for the second bore. It will be notedthat the larger holes bored in the crankcaseoverlap,

This is necessary to accommodate the synchro-nising gears and to provide communicationbetween the two portions. Owing to this overlapthe crankshafts have to be removed from theirhousings before these are removed from thecrankcase. In the more usual design the housingwith crankshaft in situ can be removed from thecrankcase. No further facing up of the externalsurfaces was attempted. Using the other set oftapped holes, the crankcase was turned through90 deg. and the holes to receive the bottom endsof the cylinder liners were similarly bored. Thecentre distance in this case being 31/32 in. Atthis stage the clearances for the connecting-rodswere milled. The holding-down lugs wereformed on the ends of the crankcase by millingwith the work held in the vertical slide.

*A Split-Single Two-Stroke Engine

An efficient unit for propelling a class s " C ”Hydroplane

by R. E. Mitchell

THE two mainjournal hous-

ings were the nextcomponents to bemade and areshown in Figs. IIand 15 . Thesewere machinedfrom 1-1/2 in. dia-meter duraluminbar.

A s u i t a b l elength was cut offand bored andreamed, longitu-dinally, to a dia-meter of 3/8 in.r ight through.At this setting theexternal diameterswere also turnedtogether with thebore to receive thesmaller ball - raceatend.

the flywheelThe work

was then reversed. .

forming the recessto take the largerball-race also atthis end. Behindthe ball-race wasmachined a smallrecess to aid in itsremoval shouldthis become neces-sary.

T h e f l a n g e swere marked offfor No. 43 dia-meter ho les totake 8-B.A. screwsto secure thehousings to t h ecrankcase. Theflywheel clearanceon the longer com-ponent was mach-ined by mountingit eccentrically onthe faceplate bymeans of a through-going bolt. Theball-races were

and placed on a 3/8 in. diameter mandrel prior tomachining the register at the crankcase end and

inserted by heating the housings up to 100º C.or so, at which temperature the races dropped

*Continue d .from pag e 506 , “ M,E,.” April 17,into position to be held tightly on cooling.

1952.Figs. 13 and 16 show the port belt which was

started as for the crankcase in that a block of

CLEAR S E C O N D FLYWHEEL

Fig. 15. Main journal housings.

548

THE MODEL ENGINEER

duralumin with square flat surfaces was firstprepared. One face was marked off for thecylinder liner bores and the transfer passages,which consist of nine 3/16 in. diameter holes, weredrilled on a 31/32 in. pitch circle diameter todepth so that the bottom of the holes coincidewith the top edges of the transfer ports in thecylinder liner. The bottoms of these holes wererounded off by grinding the drill hemisphericalto ensure smoother gas flow through the passages.It will be noted that the tenth hole which comes

APRIL 24, 1952

stresses, without causing ovality on release,when boring the cylinders by mounting in avee block on the boring table . Consequently,the liners were machined at one setting by cuttingthe cast-iron rod o f sufficient length to use oneend as a chucking-piece. The outside diameterwas made of such a size so that they were a fairlytight fit in the bores in the port belt previouslymade to receive them. This was done by trialand error while the portion beyond the flanges,where this method could not be employed,

TAPPED 4BA

3,“” OEEP

t---

2-1/2"

Fig. 18. Cylinder cooling fins

directly between the two cylinders has beenomitted to ensure that the centre distance be-tween them is as small as possible. A suitableblock of duralumin was similarly prepared for thecylinder cooling fins and is shown in Figs. 17and 18. This, together with the port belt, wassecured to the faceplate and the two bores totake the cylinder liners were machined to adiameter of 0.860 in. It is essential that thecentres are exactly the same in the port belt asin the block for the cylinder fins. The boring ofthe hole to take the transfer cylinder liner cutsthe holes previously drilled for the transferpassages so that one side of these passages isformed by the outside wall of the cylinder liner.The object of the lands between the transferpassages is so that the bottom end of the linershall be adequately supported. The bore forthe exhaust cylinder was machined after slidingthe sandwich along the faceplate by the requiredamount. No facing of the ends was attempted,since the bores are automatically at right-anglesto their ends.

The cylinder liners are shown in Figs. 19 and20 and were machined from 1-1/8 in. diameterI per cent. nickel cast-iron bar. Since thebore is 0.740 in. diameter, the wall was notconsidered sufficient to withstand the clamping

550

was made exactly the same size. It will beremembered tha t the bores in the port beltand finned block were machined at one settingwhile clamped together. Fine circumferentiallines were scribed in the lathe to mark theupper and lower limits of the ports and dividinginto ten equal parts by fine longitudinal lines.After parting off to the correct lengths theliners were mounted, separately, on a mandreland clamped securely endwise up against ashoulder. One side of the collar of one linerwas then machined, and without moving thelongitudinal position of the tool, the similarface of the collar of the other liner was facedup. This method was repeated for facing theother sides of the two collars. This ensuredthat the thickness of these is exactly the same,although the actual dimension is not of greatimportance. The bores of the liners were thenlapped, using non-expanding aluminium laps,two of which were required ; one for roughingand the other for finishing. Moles were nextdrilled for the ports using a drill, the diameterof which is slightly less than the depth of theports. This was carried out by gripping theliner s endwise in a vice on the vertical slide,centring with a centre drill and using the three-jaw chuck to hold the drill. This method, it

h:tctl-biC CCSW

25al1e

r3s.leela:riear:ale:de,at:n6agxt

h”fhele,:e-it

THE MODEL ENGINEER APRIL 24, 1952

Fig. 19. Cylinder liners,

has been found, gives more accurate locationto the holes and, if a fine feed is used, lessensthe amount of burr caused by the drill on thebreak through. It also obviates the need forcentre-pops which are impracticable on thincast-iron shells. , The circular holes so formedwere squared out by hand, using a small square

pistons and connecting-rods

file leaving a land of about 1/16 in. between eachport. The bottom external edges of the transferports were bevelled off at about 45 deg. to ensuresmoother gas flow. The cylinders were finished offby removing the slight burr, raised from the filing,with a piece of fine emery-papcr on the finger.

(To be continued)

Fig. 20. Cylinder liners

551

*A Split-Single Two- troke EngineAn efficient unit for propelling a class " C "

Hydroplane

by R. E.. Mitchell

HE pistons were next on the list and areshown in Figs. 19 and 21. These were

machined from cast magnesium alloy bar. Thismaterial has been used in all previous engines andhas the advantage over the aluminium alloys inthat its density is considerably less.

This enables the reciprocating weight tobe kept to a minimum, but against this is thepoor corrosion resistance of magnesium alloysbut it appears to be adequate. The transferpiston is comparatively short to ensure a goodgas flow from the crankcase to the transfer ports.In this design it does not matter if the transferports are uncovered at top dead centre. Theinside of the piston is machined away 2s muchas possible to reduce weight. The exhaustpiston is made slightly longer than the strokein order to accommodate the crankcase com-

*Continued from page 551, “ M.E.,” April 24,1952.

pression ring at the bottom of its skirt. Sincethe M.P.B.A. rules demand a silencer, no attemptwas made to increase the volumetric efficiencyby allowing the piston to uncover the exhaustport at top dead centre. The exterior of theexhaust piston is not in communication withthe crankcase, so the weight reduction in thiscase was carried out on the exterior of thepiston. Leaving the interior of the piston solidexcept for the connecting-rod slot helps toincrease the crankcase compression ratio. Theclearance between the piston and the cylinder israther more than usual being of the order of0.002 in. Since the ports are operated by pistoncrowns, thc compression rings, of which thereare two per piston, are placed as near 2s possibleto the tops of the pistons. The piston skirts,since they play no part in isolating the transferports from the exhaust ports, are considerablyrelieved except for lands at the top and bottom.The piston ring grooves are machined by using

THE MODEL ENGINEER MAY I, 1952

thickness after which it is slotted and thencompressed on to a mandrel with the gapclosed before turning the outside diameter tothe correct size. Fig. 22 shows the gudgeon-pins which are a straight forward turning jobfrom 3 per c e n t nickel-steel bar and are3/16 in. diameter with a 1/8 in. diameter reamedhole through the centre. No end pads areused. In a previous design serious scoring ofthe cylinder bore had resulted from their use.To provide endwise location, the piston andpin are drilled 1/32 in. diameter at each endto take a piano wire pin. The gudgeon-pinswere finally case-hardened.

From the components so far finished theconnecting-rod centres were checked. It willbe noticed that the big-end eyes are not sym-metrical. This is to provide clearance for thebalanceweights on the crank discs. Also, theweb is not symmetrical to allow clearancebetween the rod and the crankcase which ismore on one side than the other due to theemployment of desaxe cylinders. The con-necting-rods, as shown in Figs. 19 and 22,are made from duralumin.

Fig. 21. Pistons

a parting tool, leaving a groove whosewidth is 0.033 in. This tool is usedfor piston ring grooves only. The obtuseangles for the conical crowns were leftwhen parting-off the pistons. The holesfor the gudgeon-pins were drilled andreamed 3/16 in. diameter by mounting thepiston on an angle-plate on the verticalslide. It is very important that thegudgeon-pins are at right-angles to theaxis of the piston, and careful settingup is required so that this condition isrealised. The piston rings, which areof about 1/32 in. square section, are ofI per cent. nickel cast-iron as used forthe cylinder liners. They were preparedby the usual method of roughing out ablank which is lapped to the correct Fig. 22. Connecting-rods and gudgeon-pins

THE MODEL ENGINEER MAY I, 1952

The holes for the big and little-ends werefirst bored and reamed, taking great care toensure that they would be parallel to one another.Excess metal was removed by milling andfiling. The usual “ I ” section is not given tothe web owing to the need for keeping thisas small as possible. The connecting-rods, itwill be noticed, are mirror images of eachother.

IAt this stage, the components so far made were assembled and the dimensions were checked

for the rotary valve assemblies which are shownin Figs. 23, 24 and 25. The end plate consistsof a rectangular plate of dural 3/16 in. thickand machined on all faces ; the external dimen-sions, at this stage, being slightly larger thanthose of the crankcase. Two 5/16 in. diameterreamed holes were made in the required positions

and at the same centre distance as the crank-i;J shafts. These were made by mounting the work

:on the vertical slide and using the cross-slide

~L-.-__?C____~:

/ index to ensure the correct centre distance./ The stationary parts of the valves is a straight-

_~~_ forward turning operation with the outsidediameters being finished to size by mounting

Fig. 24. Crankcase backplate for rotary inlet valves on a 3/16 in. diameter mandrel. It is very impor-

I ?

Fig. 25.

558

7-

THE MODEL ENGINEER

tant that the valveface is exactly atright-angles to thebore, and absoluteaccuracy cannotbe guaranteed bydrilling a 3/16in.diameter hole ofthis length. Themajor diameterswere made about0.0005 in. smallerthan the crankcaseb o r e s previouslymade to receivethe rotary valves.Since these boresoverlap, a flat hadto be milled one a c h t o enablethem to be fitted.

MAY I, 1952

was removed andthe diameter wasr e d u c e d b y0.0005 in. An oilhole was also pro-vided for extralubrication. Oneof the spindleswas allowed toextend for 3/8 in,beyond its bearingwith the idea ofproviding for amagnero drive.T h e o t h e r i slocated endwiseby a “ C ” washerin a circumferen-tial groove andr e t a i n e d b y ascrewed cap.

I-

i

i

IL

-rJ”8

*._

If a slight error has been made in the centresof the two 5/16 in.diameter holes in the end-plate, a little scraping can correct this becausea gastight joint does not have to be made here.The rotating parts of the valves have mild-steel discs on 3 per cent. nickel steel spindles.These were connected together by screwingand riveting over before finally turning tosize. The whole of these parts would havebeen made from nickel-steel had a piece ofsuitable size been available. It was consideredthat the extra’strength of nickel-steel over mild-stee! would be needed on a spindle of only3/16 in. diameter. It must be emphasised thatsuitable radii should be left at all changes ofsection and given as good a finish as possibleto reduce the risk of fatigue failure. It maybe mentioned here that originally the spindleswere made what was considered to be a goodrunning fit in the bearings, although possiblyon the tight side with the hope that they wouldrun themselves in. On first running the engine,however, one of the spindles seired, althoughnot seriously. The pick-up on the spindle

The cut-away portions of the valves wereremoved by saw and file. It was intended thatbalancing should be effected by drilling threeblind holes nearly through the thickness of thedisc and each was counterbored slightly toreceive a disc of brass 0.010 in. thick which wassoft-soldered in position, so as not to reducecrankcase compression. This, however, d idnot achieve complete balance, so as muchmetal as ‘possible was filed away from thisregion, and even now they are not completelybalanced.

The flywheels, which are shown in Figs. 26and 27, were turned from medium carbon-steelsalvaged f r o m an old drilling machine spindle.The diameter of the wheels is governed bythe centre distance of the crankshafts, and doubtwas expressed as to whether they would beheavy enough at 4-1/2 oz. each and a diameter of1-5/8 in. Subsequent running has proved that thefears were groundless. Most of the weightis concentrated in the rim and the back side iscounterbored to place as much weight as possibleover the bearing and to prevent overhang. The

569

THE MODEL ENGINEER

bores are made taper with a total includedangle of IO deg. The groove for the startingcord is machined integral with one flywheel.Most starting pulleys appear to be made toolarge in diameter to give the engine the requiredspin to start it, but if the’ groove is made withan angle of not more than 45 deg. no slippingof the belt is experienced. T h e collets a r emachined from 3 per cent. nickel-steel left innormalised condition and have a IO deg. taperto correspond with the flywheel bores. Theyeach have one longitudinal split. The clampingnuts, one of which, of course, has a left-handthread, also form the female portions of theinboard universal joints and are case-hardened,but the threaded ends are tempered back slightly.This was carried out by standing the nut in water,covering the portion to be left hard and playinga flame on the part above the water line. Thethreads are 3/8 in. diameter x 32 t.p.i. and werecut in the lathe. When first running the enginewith fixed advanced ignition timing, the flywheelsoccasionally turned on the crankshafts due mainlyto the violent acceleration made possible withthe rather light flywheels. Owing to the samecause, similar difficulty was also experiencedwith the magneto rotor and couplings. Sinceusing a glow plug, however, this trouble hasnever been in evidence because. it is possible tostart the engine on a much more nearly closedstrangler setting.

Figs. 17 and 28 show the cylinder-head whichwas cut from solid dural and is machined all

MAY I, 1952

over. Clearance was machined for the conicalpiston crowns at the required centre distance.Exact measurement here is unnecessary. Toconnect the two cylinder bores a passage wascut by hand and is approximately 1/2 in. x 1/8 in.cross-section midway between the two cylinders.The head was reversed and drilled and tapped1/4 in. x 32 t.p.i. to take 5/32 in. reach plugs.The plug joint seating was machined at thesame setting. It will be noted that the plugis offset by 1/8 in. towards the transfer cylinder.This was done because the mixture in thisvicinity will in all probability be less dilutedwith exhaust gas, making for easier starting andhigher flame speeds through the mixture. Thelast operation on the finned belt was to machinea lug on each end which is tapped 4 B.A. Theseare to take tie-rods to enable the engine to bemore rigidly held in the hull and to distributesome of the load imposed on the crankcase hold-ing-down lugs, the bolt centres being onlyabout I in.

The induction ports in the end cover of thecrankcase are each 1/4 in. diameter, which may beconsidered small by modern standards. Thereis, however, sufficient metal for these to beopened up although this has not yet beenattempted. It was rather a problem to designa suitable induction manifold. Two carburettorswere considered but were abandoned becauseone would foul the magneto if it was ultimatelydecided to use this form of ignition.

(To be continued)

The Beauty of OldEllis. (London :

Trains, by C. HamiltonGeorge Allen & Unwin

Ltd.) 148 pages, size 6 in. by 9 in. Eightcoloured plates, 60 halftone reproductions.Price 20s. net.

For the Bookshelf

This is a most unusual book, but one that canbe read and enjoyed by anyone who knows thefascination of a railway train. To most of theolder generation, however, railway trains havenow lost much of the beauty that was once theirs,

‘and Mr. Hamilton Ellis has done well to recalland to give something like permanent record tafacts which were in danger of becoming lost.He does it most successfully in this, his latestcontribution to railway literature, by recountinghis early impressions and the modifications whichhe later thought necessary to make in them.

His well-known style of writing is much inevidence; though he wisely leaves his readersentirely free to decide whether or not they agreewith his opinions. But he is always entertainingand frequently amusing.

The eight coloured plates are worthy additionsto the still-too-rare visual evidence of the coloursof old trains and locomotives, and most of thesubjects are uncommon. The photographs,from which the sixty halftone illustrations (which

570

are printed on twelve inserted art-paper pages)have been prepared, are just right for such a bookand were obviously selected with the greatest care.We note two slips in the captions : first, the engineseen in Fig. 35 cannot be the Albert Edward;the trailing hornblocks show that she was originallya 2-2-2 later altered to 4-2-2. Although it isdifficult to decide precisely from scrutinising ahalftone reproduction, the number appears tous to be 3021, which would mean that the engineis actually the old Wigmore Castle, one of thisclass which was originally built on the 2-2-2wheel arrangement. The second slip concernsFig. 54 ; the 4-4-2 tank engine seen in thispicture must be No. 1126 ; the G.C.R. locomo-tive bearing the number 126 was a 0-6-0 tenderengine designed by Parker and built about 1894

We are interested in Mr. Ellis’s commentsconcerning the marked similarity between theGreat Northern locomotives and stock, and thoseof the Great Northern of Ireland. The fact isthat J. C. Park, who introduced English Gre tNorthern features to the G.N.R.(I) was Bformer ya draughtsman under Patrick Stirling at Don-caster ; Mr. Ellis was evidently unaware of this.

All the same, this delightful book seemsdestined to find a place among the classics of itskind.

*A Split-Single Two-Stroke Engine

An efficient unit for propelling a class " C "Hydroplane

by R. E. Mitchell

ANOTHER objection to twin carburettors isdue to their being displaced laterally by

27/32 in. Due to centrifugal force, fuel feed to theinner carburettor may cease altogether, and wouldnot probably be compensated for completely bythe other, thus upsetting the mixture strengthafter a certain speed had been reached. A mani-fold in the form of a “Y,” using a single car-burettor, was considered but had to be abandoned,again because of fouling the magneto. If this

is 1/32 in. diameter with two similar holescommunicating with the choke. This type ofjet is not usually considered to be the best fromthe performance point of view. The moreusual type whereby only the tapered portion ofthe needle obstructs the gas flow has since beentried, but an improvement in performance wasnot detected. One disadvantage, however, wasnoted in that the needle setting for optimumresults was made far more critical ; too critical

Fig. 28. The cylinder-head

_

Ii’

_

could have been avoided the carburettor toonear the magneto would probably result in thelatter being sprayed with fuel. Although notideal from the gas flow aspect, the final designdecided on is as shown in Figs . 29 and 30.The carburettor is cut from the solid and is ofdural. The choke is of oval cross-section andmeasures 19/64 in. x 1/4 in., the major dimensionbeing in the horizontal plane. Since with a two-stroke engine having rotary disc inlet valves,the carburettor is very low in the hull, makingit rather inaccessible, it was decided to use a jetwhich can be removed by unscrewing a nut ontop of the venturi. The jet assembly is bestshown by an illustration and is shown in Figs. 29and 3 0 . It is made of brass and the jet proper

*Continued from page 570, “ M.E.“, M a y 1,1952.

to ensure repeatable performance and wasabandoned. The needle is of 1/16 in. diametersilver-steel soft-soldered into a brass nut whichis milled on the outside to engage with a bronzeleaf spring anchored to the carburettor bodywith two I O-B.A. screws. Some designs rely onthe friction of tight screw threads to hold thejet needle in position, but this is considered tobe a rather crude method, being very susceptibleto movement due to engine vibration.

Most engines of 10 c.c. capacity and less arenot fitted with any means of controlling theirspeed. In aircraft practice the propeller does,however, provide a suitable load. One of thesurest ways of wrecking an engine is to allowit to attain the high speeds of which it is capablewithout a load. Under these conditions thecentrifugal loads applied, particularly to theconnecting-rod, are far in excess of the normal

THE MODEL ENGINEER MAY 8, 1952

Fig. 29. The carburettor

working stresses. This can easily be avoided equally well in the present two-stroke case.by the fitting of a simple strangler consisting of The usual idea that a glow-plug engine isa disc pivoted so that it can be moved across the capable of little or no speed control has beencarburettor inlet, thus obstructing the gas flow. found to be erroneous. Poor carburettor designThis method has been used successfully on four- is likely to be mostly responsible. Since thestroke engines using spark ignition, and works boat in which this engine is installed is fitted with

Fig. 30. Carburettor details

.

THE MODEL ENGINEER

a device operated by water pressure in thecase of submergence, the strangler disc and itslever are independent although pivoted on thesame spindle, as shown in Figs. 29 and 31.Connection between the two is made by meansof a peg which is withdrawn when the knock-offdevice is operated, so allowing the spring-loadeddisc to cover the carburettor intake and prevent-

MAY 8, 1952

into the dural do strip, most of the holes being atleast three diameters deep, there is sufficientspace for them to be tapped out to a larger size.Dismantling an engine except for repairs oralterations should not be carried out, becausethe alignment is bound to be adversely affectedon reassembly.

All the joints, which are fairly wide, are metal

ing a charge of water entering the crankcase.Previous experience with water entering a highrevving engine has shown that the results canbe serious. This is also used as the normalstopping method, and although instructionleaflets accompanying several commercially madeengines state that this method must not beused, no harm appears to have arisen from itsuse. As a matter of interest, the same methodhas been used for stopping a four-strokeengine with a compression ratio of 14-1/2 : Iwhere the chances of a hydraulic lock are muchgreater.

The drilling and tapping of the holes requiredfor holding the various components togetherwas the last oneration. The housings for themain bearings are each secured to the-crankcaseby five screws, the rear crankcase cover by ninescrews and the stationary parts of the rotaryvalves by five screws each. All these screws are8 B.A. and have cheese-heads. The port beltis secured to the crankcase by four through-going6-B.A. hexagon-head screws and two 6-B.studs. The studs are used beneath the exhaustports. The cylinder-head is held down bytwelve 8-B.A. cheese-head screws which passthrough the finned belt into the port belt, theholes being deeply counterbored to receive theheads. The use of screws in preference to studsand nuts may be regarded as taking the easyway out and is the first design in which theyhave been used so extensively. Studs have beenused in a four-stroke design from which thenuts have never been removed in the four yearsof its existence. If, however, the threads tapped

598

to metal, no gaskets or jointing materials whateverare used.

When first attempting to start the engine amagneto driven from one of the rotary valvespindles was used. The engine stopped after afew revolutions and the trouble was found to bedue to the magneto coupling having slipped onthe shaft. The flywheels are rather on thelight side and the initial acceleration was probablythe cause of the trouble. Attempts to remedythe defect failed, and since the power boat seasonwas approaching, the spark ignition was aban-doned in favour of a glow-plug. Fig. 17 showsthe glow-plug connector which is an improve-ment on the usual crocodile clip. It consists ofa piece of 1/4 in. thick ebonite sheet providedwith two bronze leaf springs which grip thecylinder-head. It is located laterally by theends of the screws securing these springs en-gaging in the cylinder-head cooling fins. Theswitch also contains two bronze spring contactswhich normally close on the live terminal of theplug. To disconnect the plug from the battery,the contacts are opened by a cam placed betweenthem.

A 2 V Exide “ Gel-Cel ” accumulator is usedand the length of the leads is adjusted so that thevoltage at the plug when on load is 1.5 V. Withthis arrangement, no glow-plug has been burntout during the whole of the season.

After running the engine for a few minutesone of the rotary valve spindles seized. Thiswas easily removed by warming the dural bush.The “ picked up ” metal on the spindle wasremoved and the valve was replaced. The

eSld

rvrdtlV

rt:te

THE MODEL ENGINEER MAY 8, 1952

Fig. 32. Engine installed in the hydroplane ” Gamma ”

engine a p p e a r e d t o behave itself , but thespeed of the hydroplane showed signs ofdropping.

This was at first thought to be due to pistonring flutter on account of the light ring pressureswhich were being used. Replacing these withrings having a higher pressure resulted in nodetectable improvement. However, when tryingthe engine with new piston rings, the same rotaryvalve spindle again seized. Both spindles wereremoved and a further 0.0005 in. was taken offtheir diameters. The speed of the boat wasthereby increased and no further trouble wasexperienced on this score.

No attempt has been made to determine thepower output, since a dynamometer to absorbthe power from the two shafts would be difficultto devise. To take the power from one shaftonly would mean that the whole of the outputfrom one cylinder would be transmitting throughthe synchronising gears, a thing which it isdesirous to avoid. Also, no attempt has beenmade to measure the engine speed when runninglight, since this only imposes undue stresses onthe engine to no purpose.

The final photograph, Fig. 32, shows a planof the engine as installed in the hydroplaneGamma.

A High-Pressure Table-Engine(Continued from page 595)

and links, the governor controls a butterfly-typethrottle valve mounted immediately in front ofthe steam-chest.

FinishAll bright parts of the model are polished to a

high degree, using various grades of emery downto “ blue-back,” and plenty of that usefulcommodity, elbow-grease. Other parts are apleasing shade of green, with a stone-colouredfoundation and stained and polished base-board.

Normally a glass case is used to cover themodel, but it may be worked under com-pressed air.

Speaking personally, my chief regret is thatI have not had the pleasure of seeing it working.It must be a very pleasant sight, at that statelygait of one revolution in two seconds, and to me,at least, would be worth twenty times the sightof a buzz-box batting round an arena at ninetymiles an hour ! It’s a good job we aren’t all alike,isn’t it ?

599